Ethylenimine (EI) is an active three-membered cyclic amine and is a very useful compound since it can introduce an amino group by an addition reaction, substitution reaction, ring opening reaction and the like. Ethylenimine is especially important as an aminoethylation agent of compounds containing an active hydrogen. It is also useful as a monomer for polyamine-type polymers in homo and co-polymerizations. In addition to all of these uses, it is also possible to prepare derivatives which retain the ring opening reactivity of ethylenimine through an addition reaction of the amino group. All of these features make ethylenimine an important substance both chemically and industrially.
Ethylenimine can be synthesized by one of several methods. One is the Gabriel method in which a beta-halo-ethylamine undergoes a ring closure through a treatment with a concentrated base or silver oxide. Another involves the reaction of ethylene chloride (1,2-dichloroethane) with anhydrous ammonia in the presence of a base. This reaction and equivalent reactants for form EI and substituted EI's are disclosed in U.S. Pat. No. 3,336,294. Yet another preparation of EI involves a decomposition (ring closure) of monoethanolamine sulfuric acid ester by hot concentrated base. Each of the above methods present certain disadvantages. For example, it is necessary to control the reaction conditions strictly to synthesize both beta-haloethyl amine and monomethanolamine sulfuric acid ester. The syntheses tend to be accompanied by side reaction and side products. All of these problems make these starting materials very expensive. At the same time, the halogen and sulfuric acid ester group which are introduced in the syntheses are removed in the subsequent process making these syntheses wasteful from the stand point of the functional group utilization. Furthermore, both processes use a base for the ring closure reaction. The bases most often used are sodium hydroxide and potassium hydroxide and these bases are used as concentrated solutions in large quantities. Thus the base requirement per ethylenimine unit is very high and uneconomical. The by-products, NaCl, Na.sub.2 SO.sub.4 or the potassium equivalents, are a further expense since they have little value and must be disposed of. The lost chlorine values in the method using 1,2 dichloroethane makes this process an expensive one. None of the art processes are readily made continuous so as to be more attractive commercially.
A more recent process involving the vapor phase dehydration of monoethanolamine is disclosed in Japanese Patent Publication No. 50-10593/1975. A catalyst of tungsten oxide alone or preferable with another metal oxide as an assistant is employed. The metal oxide assistant includes lithium, magnesium, tin, bismuth, molybdenum, nickel and aluminum oxides.
Such assistants or promoters may be added to the catalyst in known manner by depositing a promoter metal or oxide on the catalyst support either prior to, coincidentally with or after the deposition of the catalytic material. The reaction is conducted at a temperature of 350.degree. C. to 450.degree. C. preferably using an inert diluent gas such as ammonia or nitrogen. Conversions of up to 45% and selectivities of as high as 66% are reported.
It has now been discovered that a coating of silica over a tungsten oxide catalyst improves its life and maintains the selectivity and conversion close to the 50% level. Without the silica addition both selectivity and conversion fall off rapidly.